French Franc Stamps Still Being Used As Postage

My son ordered a watch battery off the internet
and received it enclosed in this envelope.
It's franked with one 1998 3 Franc + 60c (charity surcharge)
and a 1981 1.40 Franc stamp.



I'm aware of the fact that old obsolete Franc stamps
are allowed by the French Post but at the proper
conversion rate.
As of Jan 1, 1999 the fixed exchange rate with the euro
was 6.55957 FRF for 1 euro.
So the seller stuck on a total of 4.40 Francs = 0.67 Euro
or about $1 Canadian.
Obviously way underpaying the proper rate.
Note also that the 1.40 was probably already used
on mail but not cancelled.

The only explanation I can think of is that perhaps
the postal clerk thought these were Euro stamps.
But if this is widespread in France then the French Postal
Service must be in the red.

French postal employee: J'ai la flemme.